This invention relates to improvements in forming containers of nitrile thermoplastic material and more particularly to process improvements rendering the containers more suitable for packaging environmentally sensitive materials such as foods, beverages, pharmaceuticals and the like, as well as the resulting containers and preforms for forming same.
It is known and desirable to form containers such as bottles, cans, tubs, trays and the like from shaped preforms by distending them while at molding temperature in a mold cavity into container form. In packaging environmentally sensitive or disintegratable materials such as, for example, foods, beverages, pharmaceuticals and the like, it is also known to use polymers comprising at least about 20 weight percent polymerized nitrile-group-containing monomer because of their resistance to oil and grease as well as their attractive oxygen, water and strength properties. In forming and processing these and other synthetic polymers, trace amounts of unpolymerized residual monomer are invariably present which can be carried over into the end container product and this can present problems if such monomers are deemed hazardous to health and/or safety by regulatory agencies in the sense of diffusing out of the plastic into the contents of the package.
As noted in U.S. Pat. Nos. 3,870,802 and 3,974,247, nitrile polymer resins can also contain trace though detectable amounts of hydrogen cyanide (HCN) which may be effectively scavenged by having a formaldehyde, styrene oxide or like compound in contact with the resin during forming thereby avoiding imparting undesirable taste otherwise caused by such HCN to environmentally sensitive contents intended for human consumption which are packaged in such resins.
It is likewise known before this invention that exposure of bottles formed of such nitrile resin to low dosage electron beam radiation is effective in reducing residual acrylonitrile monomer by causing it to directly polymerize in place in the solid polymer walls. However, it has also been determined that such exposure causes undesirable side reactions of which generation of excess HCN is predominant, even with HCN scavengers present in the polymer at the time of radiation exposure. Thus, though a promising approach exists toward solving the problem of unpolymerized nitrile monomer in nitrile thermoplastic materials, another problem has developed requiring solution if the attractive packaging properties of these resins are to be further exploited.